In welding of stainless steel, flux cored wires are widely used since they have high welding speeds and have low spatter and thus requires little cleaning operations after welding. This flux cored wire is a composite wire comprising a shell made of mild steel or stainless steel with a powder filling of compounds such as metals, oxides or fluorides. In welding, this flux cored wire is supplied to a weld bead as a consumable electrode while an arc is successively generated to form molten metal.
When the flux cored wire is fused by the heat of arc at a high temperature, metallic components, oxides and fluorides are evaporated from the molten metal to produce fumes, which are particles connected in the form of a chain. It is known that in this welding environment, when a worker inhales scattered fumes, he/she develops metal fume fever or other problems. In particular, in welding of stainless steel, chromium contained in welding materials and stainless steel base material are contained in fumes in a high level, but a part of the chromium content is present in the form of hexavalent chromium, which is suspected to be carcinogenic and have other risks. Therefore, it is particularly harmful for human body. For this reason, it is necessary to prepare special equipment to keep workers from inhaling fumes during welding operations, which lowers welding workability.
To this end, a technique for suppressing leaching of hexavalent chromium from fumes by controlling the amount of Si, Ti, Zr, Na, K and Cr contained in a wire has been conventionally suggested (Japanese Patent No. 03765772). However, this prior art has the disadvantages of high spatter during welding and low slag removability, leading to poor welding workability. For this reason, this prior art has not been put into practical use. Techniques for suppressing leaching of hexavalent chromium from slags generated during welding have been suggested (Japanese Unexamined Patent Publications Nos. 2008-246523, 2009-154183, 2009-154183). However, these two prior art techniques cannot suppress leaching of hexavalent chromium from fumes. There are some prior art documents that discuss adding polytetrafluoroethylene (hereinafter referred to as PTFE) to a flux of a flux cored wire (Japanese Unexamined Patent Publication Nos. 2007-90376, H6-155080), but the object of these prior art documents is to reduce the amount of diffusible hydrogen of a weld metal of carbon steel and prevent weld flaws, and they are not directed to leaching of hexavalent chromium from fumes since almost no Cr is contained in the fumes.
As stated above, known stainless steel flux cored wires have not been successful in sufficiently reducing the amount of water-soluble hexavalent chromium contained in fumes generated during welding of Cr—Ni-based stainless steels such as SUS304 and SUS316, Cr-based stainless steel such as SUH409 and SUS430 and other materials.